Broadband access technology mainly includes copper access technology (such as various digital subscriber line (DSL) technology) and optical access technology. The access network implemented by optical access technology is referred to as Optical Access Network (OAN). Based on different network deployment methods, OAN is classified into Fiber to The Home (FTTH), Fiber to The Building (FTTB), Fiber To The Business (FTTBusiness), Fiber To The Curb (FTTC) and Fiber To The Cabinet (FTTCab).
The Gigabit Passive Optical Network (GPON) is the latest technological standard for passive optical network (PON), and ITU_T Standard of GPON corresponds to a series of G984.1, G984.2, G984.3 and G984.4.
FIG. 1 shows the architecture of a GPON protocol stack, which is described as following:
A GPON Transmission Convergence (GTC) layer, includes two sub-layers:
TC Adapter Sub-layer, which, during downstream, is adapted to cut service data received in Asynchronous Transfer Mode (ATM) into ATM cells and cut service data received from a client in a GPON Encapsulation Mode (GEM) into GEM data blocks, and during upstream, adapted to assemble ATM cells or GEM data blocks in GTC frames into corresponding service data;
GTC Framing Sub-layer, which, during downstream, is adapted to assemble GTC TC frames, i.e., add CTC TC frame head ahead of the ATM cell or GEM data block based on control information of physical layer operation and maintenance (PLOAM) so as to form a complete GTC TC frame, deliver the GTC TC frame to a GPON Physical Media Dependent (GPM) layer, and during upstream, adapted to receive GTC TC frames from the GPM layer, remove the frame head, and deliver the GTC TC frames to the GTC Framing Sub-layer for further process.
A GPON Physical Media Dependent layer is adapted for GTC frame transmission through optical fiber;
A PLOAM physical layer is adapted for operation, management and maintenance of PON physical layer;
ONU Management and Control Interface (OMCI) is adapted for the control of OLT over ONT, wherein the data from OMCI is encapsulated into ATM cells or GEM data blocks for transmission in the same way as ordinary service data.
The GTC layer of GPON provides two modes for encapsulating service data, i.e., Asynchronous Transfer Mode (ATM) and GPON Encapsulation Mode (GEM): the ATM is to encapsulate the service data into ATM cells with 53 bytes; the GEM is a variable length encapsulation, which is capable of changing the length of a GEM encapsulation frame according to the length of service data frame.
Service data transmission includes unicast transmission and multicast transmission. The unicast transmission is point-to-point transmission and the multicast transmission is point-to-multipoint transmission. A PON system inherently supports point-to-multipoint transmission during downstream; therefore, a GPON system is advantageous in supporting multicast services, such as IP multicast services carried over Ethernet.
Before transmitting data, optical line terminal (OLT) and Optical Network Terminal (ONT) first negotiate a service transmission channel through OMIC control messages. The service transmission channel of ATM is referred to as Permanent Virtual Circuit (PVC), and the service transmission channel of GEM is referred to as GEM port. OLT assigns a Virtual Path Index (VPI) and Virtual Channel Index (VCI) of PVC for the ONT in an ATM mode; OLT assigns a port ID of GEM port in a GEM mode. AVPI/VCI and a port ID are globally unique, i.e., different VPIs/VCIs and port IDs are assigned for different service flows.
As for downstream unicast data transmission from OLT to ONT, since the data transmission from OLT to ONT in a PON system is in a broadcast mode, ONT receives data carried by all the ATM PVCs or GEM ports sent from OLT at TC Adapter Sub-layer of protocol stack. The ONT may not necessarily need to receive all the data. Accordingly, the TC Adapter Sub-layer of ONT provides data filter function in terms of ATM PVC or GEM port: OLT assigns a VPI/VCI or a port ID corresponding to the ONT for downstream data, and the ONT only receives downstream data carried by the corresponding VPI/VCI or a GEM port with the corresponding port ID, which are illustrated in FIG. 1 as VPI/VCI filtering entity and port ID filtering entity in the GPON protocol stack.
As for upstream unicast data transmission from ONT to OLT, ONT carries data in an ATM PVC or a GEM port according to VPI, VCI (in an ATM mode) or port ID (in an GEM mode) assigned by the OLT, and transmits the data within a specified transmission time window based on transmission time windows assigned by the OLT.
In the following description, a GEM mode is taken as an example to describe an embodiment of the present invention. It should be noted that the whole description for a GEM mode is also applicable to an ATM mode.
Transmission modes for downstream multicast data from OLT to ONT of GPON specified in G984.3 protocol may be:
1. All the downstream multicast flows from OLT are transmitted through a GEM port; or
2. The downstream multicast flows from OLT are transmitted through multiple GEM ports.
FIG. 2 is a schematic diagram illustrating the scheme that all the multicast flows are transmitted through a same GEM port. As shown in FIG. 2, the Optical Distribute Network (ODN), as a passive splitter, permits downstream data from the OLT to be transmitted to each ONU through optical splits; similarly, the ODN permits the upstream data from the ONU to be transmitted to the OLT through convergence. OLT transmits flows of multicast group G1, G2 and G3 over the transmission channel of a GEM port with a port ID labeled as P1.
FIG. 3 shows is a schematic diagram illustrating the scheme that the downstream multicast flows from the OLT are transmitted over multiple GEM ports. As shown in FIG. 3, OLT transmits flows of multicast group G1 through the transmission channel of a GEM port with port ID labeled as P1, and transmits flows of multicast group G2 and G3 through the transmission channel of a GEM port with port ID labeled as P2.
FIG. 4 shows the internal configuration of an ONT device that supports multicast services. As shown in FIG. 4, CPU in the device is configured to control various modules, and GPON protocol processing module is configured to extract a downstream multicast flow from a GTC TC frame sent by OLT and transmit the multicast flow to a Medium Access Control (MAC) bridge. The MAC bridge is connected to the GPON protocol processing module through an internal Ethernet port. The MAC bridge also provides externally visible Ethernet ports for access to multiple user devices.
According to the current standard, in case that a GEM port carries multiple multicast groups, GPON protocol processing module in the ONT entirely receives all the multicast data carried by the GEM port, and then transmits the received data to the MAC bridge through the internal Ethernet port. To prevent multicast flows from being broadcasted in the MAC bridge, Internet Group Management Protocol snooping (IGMP snooping) is used to transmit multicast flows on demand in the prior art.
As shown in FIG. 5, the ONT supports the IGMP snooping function. A user CPE1 sends an IGMP message to join a multicast group G1, and the multicast flow is transmitted only to the port which has received the IGMP message.
As shown in FIG. 6, if the user CPE1 sends an IGMP message to join an unauthorized multicast group G1, the ONT cannot filter multicast flows, and thus continue transmitting unauthorized multicast flows to CPE1.
Thereby, the prior art has the following problems:
Under the current standard, in the case that an ATM PVC or a GEM port carries multiple multicast groups, since the IGMP snooping of ONT restricts only the port which is adapted to transmit the multicast flows, it is not possible to judge whether a user is joining an authorized or an unauthorized multicast group. If the user joins an unauthorized multicast group, the multicast data can also be transmitted to the user. In other words, it fails to filter the unauthorized multicast data.